The present invention relates to the determination of connectivity faults in communication systems, especially between chips at opposite ends of serial data transmission path.
Connectivity faults are of several types including “stuck-at” faults in which a transmitted signal appears stuck at either a low state or a high state and fails to transition between states. Other types of connectivity faults include those in which a coupling capacitor used for alternating current (AC) mode operation is shorted, and those in which the arriving signal appears to “float”, such as when the cable is disconnected from the signal source. Elements which provide connectivity between chips at remote and near ends of a transmission link include at least a transmitter of the chip at the remote end, the package of the chip at the remote end, a card connecting the remote end chip to the chip at the near end, an AC coupling capacitor, the package of the near-end chip, and a receiver implemented on the near-end chip. Connectivity between chips of such system is frequently referred to as “cable connectivity”, whether or not a literal cable (rather than a card or other conductive connection) is provided between the packages of respective chips. One way to test the cable connectivity is to send a stream of test data from a remote transmitter and then verify the data received at the near-end receiver.
In AC coupled transmission links, termination is provided at the source (transmitter side) such that no DC current is sunk at the receiver end. In such AC coupling mode, a high-pass filter exists by the combination of the series AC coupling capacitor (also referred to as a “DC blocking capacitor”) and a termination resistor which is shunted to ground. The high-pass filter causes low-frequency test signals to decay with an “RC” time constant determined by the magnitude of the resistance (R) and the capacitance (C) of the respective circuit elements, which can be fixed or variable.
Differential signal transmission is frequently favored for the transmission of higher frequency RF signals over signal conductors. An advantage of differential signal transmission is larger peak-to-peak differential signal swing and improved common mode noise rejection. However, differential signal transmission poses particular challenges. Heretofore, robust systems have not been provided for detecting and isolating a single-ended connectivity failure of a differential signal transmission link. For differential signal transmission links, it is not sufficient to detect the presence and/or absence of signals on a pair of signal conductors, a robust cable fault detector must determine which of two cables carrying the paired differential signals is faulty. The challenges of testing are particularly great when detecting a failing signal conductor when communication between chips is provided in an AC coupling mode in which signals must pass through an AC coupling capacitor, because such capacitor blocks direct current (DC) transmission. While some techniques such as AGC (adjusted gain control), DFE (decision feed-back equalization), etc., are available to reconstruct received signals, such techniques are of no use when signals are suddenly ruined by signal interference, or accidental disconnection of a cable from one or two ends of the transmission link.
Conventional signal detectors operate by detecting the absence of a valid signal within a specified latency. However, a signal detector cannot determine whether the failure is due to the cable, or the data itself. Neither can it tell which cable has a problem, or what type of defect mechanism is present, i.e., whether the fault is one of stuck-at high, stuck-at low or floating. This is because such signal detectors detect the presence of absence of signal from the difference between levels of a pair of differential signals arriving from the transmission link.
The 1149.6 standard of the Institute of Electrical and Electronics Engineers (IEEE) entitled “IEEE Standard for Boundary-Scan Testing of Advanced Digital Networks”, published April, 2003 (hereinafter IEEE 1149.6), specifies a cable fault detector at a receiver end of a transmission link for detecting single-ended cable faults for both AC and DC coupling modes. The published IEEE 1149.6 standard provides an example of using a hysteresis comparator 10 (FIG. 1) to detect loss of signal on the transmission link. In the published example, the hysteresis comparator compares the level of a signal arriving from a single-ended cable of the transmission link with a delayed version of the same signal. The published example does not provide for robust testing, because when testing for a signal in AC-coupled mode, the comparator's operation destroys the hysteresis effect. For an AC coupled signal having an RC decay, as described above, when the decay drops to a certain level, the output of the hysteresis comparator 10 changes state. The hysteresis comparator 10 of the published example is poorly suited for testing an AC-coupled mode, because it does not maintain state when the AC-coupled signal decays.
Commonly assigned U.S. patent Publication No. 2005/0190828 to Hsu et al. describes a cable connectivity test receiver operable in accordance with IEEE 1149.6. However, that application does not seek to address the problems that are uncovered and addressed in accordance with the below-described embodiments of the invention. Said U.S. patent Publication 2005/0190828 is incorporated herein by reference.